Spirochetes are bacteria of major medical importance. Some of the most fundamental aspects of their biology or their mechanisms of pathogenesis are not understood. One of their unique attributes in rapid motility. A genetic-biochemical approach towards understanding how Leptospira swims has been initiated. The results obtained have been significant. Mutant analysis indicate that the axial filaments (AFs) are involved in motility. Based on these results, a model on how they swim was proposed. Micro-cinematography and an analysis of AF proteins from recently isolated motility mutants are being used to test the model. Another approach being used is an analysis of chemotaxis mutants. These mutants will be fully characterized by a capillary tube assay and a tethered cell assay developed for Leptospira. The latter will allow for sophisticated tracking experiments on individual cells similar to the ones used on rod-shaped bacteria. A novel phenomenon in the micro-cinematography study was recently observed. Antibody coated latex beads attached to swimming cells were rapidly displaced from the front to the back. The results suggest that the bead is attaching to an outer membrane antigen. This antigen is dragged laterally through the membrane by the retarding viscous forces of the medium as the cell swims in a given direction. If correct, then antigens can move laterally through membranes faster (11 Mum/sec) than previously detected. This hypothesis will be further tested using a number of techniques including monoclonal antibodies and immunolocalization. The approach used on Leptospira is being extended to the more difficult to grow, structurally complex, and medically important Treponema. Motility mutants have been isolated of T. phagedenis with no AFs. Others have been isolated with altered cell shape. By analyzing the AFs and cytoplasmic tubules from these mutants, a better understanding of the function of these structures, how these bacteria swim, and their overall biology should be achieved. The spirochete research has not been confined to motility. Radiotracer studies on Leptospira revealed the unusual finding of two major pathways of isoleucine biosynthesis being present. In addition, genes have been cloned from Leptospira, and they have been found to be expressed in Escherichia coli. The latter result will permit further studies relating to mechanisms of virulence, diagnostic procedures, and gene structure.